The present study investigated the impact of motor commands to abort ongoing movement on position estimation. Participants carried out visually guided reaching movements on a horizontal plane with their eyes open. By setting a mirror above their arm, however, they could not see the arm, only the start and target points. They estimated the position of their fingertip based solely on proprioception after their reaching movement was stopped before reaching the target. The participants stopped reaching as soon as they heard an auditory cue or were mechanically prevented from moving any further by an obstacle in their path. These reaching movements were carried out at two different speeds (fast or slow). It was assumed that additional motor commands to abort ongoing movement were required and that their magnitude was high, low, and zero, in the auditory-fast condition, the auditory-slow condition, and both the obstacle conditions, respectively. There were two main results. (1) When the participants voluntarily stopped a fast movement in response to the auditory cue (the auditory-fast condition), they showed more underestimates than in the other three conditions. This underestimate effect was positively related to movement velocity. (2) An inverted-U-shaped bias pattern as a function of movement distance was observed consistently, except in the auditory-fast condition. These findings indicate that voluntarily stopping fast ongoing movement created a negative bias in the position estimate, supporting the idea that additional motor commands or efforts to abort planned movement are involved with the position estimation system. In addition, spatially probabilistic inference and signal-dependent noise may explain the underestimate effect of aborting ongoing movement.
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